Download Freediving safety lanyard test recommendation

Freediving safety lanyard test recommendation
Aida International competition rules does not have good basis for safety lanyard tests. Currently all the lanyard
tests in Aida competitions are done by judges with subjective and various ways. Therefore, I have created this
simple lanyard safety test/check recommendation. This test pattern is based on some physical facts (calculations
by Mikko Pöntinen):
1. Minimum sudden force and drop test
The force that the drop test creates on the lanyard depends completely on how much the lanyard stretches. The
more it stretches, the smaller the force against the lanyard. The reason is that when the lanyard cable stretches
more, the weight has more time to decelerate, so the impact is not as sharp. Some examples:
5 kg drop from 1 meter:
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Lanyard stretches 5 cm: Force = 980 N (appr 100 kg)
Lanyard stretches 2 cm: Force = 2450 N (appr 250 kg)
Lanyard stretches 1 cm: Force = 4900 N (appr 500 kg)
Lanyard stretches 0,5 cm: Force = 9800 N (appr 1000 kg)
Lanyard stretches 0,1 cm: Force = 49000 N (appr 5000 kg)
20 kg drop from 0,3 meter:
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Lanyard stretches 5 cm: Force = 1180 N (appr 120 kg)
Lanyard stretches 2 cm: Force = 2940 N (appr 300 kg)
Lanyard stretches 1 cm: Force = 5880 N (appr 600 kg)
Lanyard stretches 0,5 cm: Force = 11800 N (appr 1200 kg)
Lanyard stretches 0,1 cm: Force = 58800 N (appr 6000 kg)
Drawing conclusions from those numbers, I would advice freedivers to construct lanyards that stretch a bit,
because they are stronger and won't dislocate divers' shoulders in worst scenarios.
2. Constant force
Calculating the drag force of diver when a counter ballast system is pulling him up, we can use the drag equation
where:
F is the force of drag,
ρ is the mass density of the fluid (1000 kg/m^3 for water),
u is the velocity of the object relative to the fluid (1,5 m/s for a normal counter ballast system),
A is the reference area (depends on the position and the size of the diver, but let's say it varies between 0,1 m^2
and 1 m^2), and
C is the drag coefficient (0,7-1 for a diver)
So, the maximum drag force is
D
D
Fd = 0,5 x 1000 kg/m^3 x (1,5 m/s)^2 x 1 x 1 m^2 = 1125 N = 115 kg.
We add to this the negative buoyancy of the diver, which is at worst 5 kg. So we end up with 120 kg.
120 kg drag is a worst-case scenario where a very big diver is coming up sideways (lanyard is attached to a waist
belt) at the speed of 1,5 m/s.
A more realistic drag would be:
Fd = 0,5 x 1000 kg/m^3 x (1,5 m/s)^2 x 0,7 x 0,1 m^2 = 80 N = 8 kg
Again we add the negative buoyancy to that force, and we get 13 kg. 13 kg drag is a more realistic scenario,
where a diver is coming up one hand first in a somewhat streamlined position.
Conclusions and suggestion for safety lanyard test for competition
judges:
This lanyard test should be simple and easy to be conducted for every freedivers, judges and competition
organizers. Purpose of this test is to make it possible to ensure that the competition lanyard has been properly
made and safe enough for competition conditions. The lanyard test should be also similar (=standartized) for all
the athletes, judges and organizers.
My recommendation is that the lanyard test would consist of doing 20 kg drop test from 0,3 m (=minimum length
of the lanyard, regarding current competition rules) or rather with 5 kg drop test from 1 meter, while the wrist
band is attached to a strong and unmoving bar about the same diameter as a wrist. Five kilograms’ weight is
safer for the judges/testers to handle and easier to arrange. The both tests are enough to reveal if a lanyard is
not strong enough, but every well-made and safe lanyard should withstand it.
Constant drag force test should be done for wrist velcro attachment. As we saw, the drag force is much smaller
than the sudden force, so I think every lanyard that can take either of these drops test will endure the constant
force test as well. Therefore, I am suggesting to test constant drag for 1 minute with 75 kilograms (or with the
diver´s weight / hanging on it when lanyard is attached).
Lanyard should be also allowed to be elastic to increase safety (based on the calculated facts above). Therefore in
both of these lanyard tests the length of the lanyard should be allowed to stretch maximum 25 % from its original
length.